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Lesson Note

Subject: Biology


Learning Objectives: by the end of the lesson, the learners should be able to:

1. Use the knowledge of energy losses in the ecosystem to explain the pyramid shape of feeding relationships.
2. State that only a small percentage of the radiant energy actually gets to plants.
3. State the first and second laws of themodynamics and use them to explain ecological events such as pyramid of energy , food chain, energy flow.


Energy exists in various forms, These forms of energy are inter- convertible , i.e , one form of energy can be transformed into another form, Such energy transformation are governed by the laws of themodynamics.

In nature, energy transformation is brought about by living organisms. Their activities cause energy to flow through the ecosystems. The sun is the ultimate and eternal source of energy for ecosystem on earth.


Energy flow is unindirectional non- cyclic, it is either utilized or stored. Light energy from the sun is absorbed by chlorophyll in green plants; and used in photosynthesis to produce carbohydrates; the chemical energy in the carbonhydrate is passed down the food chain when the primary consumers food on the plants producer the chemical energy is passed along the food chain to the secondary consumer and them to the tertiary consumer decomposer.


Energy is lost at each trophic level- when a herbivore/ primary consumer feeds on a plant/ producer ; not all parts of the plant is eaten ; as a result not all the energy in the plant /producer is consumed plants lose energy via respiration, and they do not utilize all the energy present in preceding members. Energy is also lost in respiration , execution , movement , and other metabolic activities.

In ecosystem, energy is also lost through the following :(i) Vegetation (ii) Soil ( iii) Air ( iv) Heat (v) evaporation of water (vi) Effects of wind

Depending on the type of vegetation and climatic factors only about 1-10% of the solar energy may be available to photosynthetic producers in most ecosystems.


Thermodynamics ordinarily means heat changes. Heat is a form of energy and how it can be changed or converted from one form to another and is governed by two laws. These laws are called first and second laws of thermodynamics.

First Law of Thermodynamics: The first law of thermodynamics states that energy can neither be created nor destroyed. In other words, the law states that energy can neither be created nor destroyed although it can be changed from one form to another.

Second Law of Thermodynamics: The second law of thermodynamics states that in any conversion of energy from one form to another there is always a decrease in the amount of useful energy. Put in another way, the law states that no transformation of energy from one state to another is ever 100 percent efficient.

Application of Both Laws to Ecological phenomena or Events

The ecological phenomena or events can be explained using the laws of thermodynamics Ecological phenomena or events in which the laws can be used to explain are food chain pyramid of energy and energy flow.

Food Chain

a. Using the first law: Energy in generated through the sun and is transferred from the producer to the final consumer. The green plant transfers the energy to the primary consumers which again transfer the energy to the secondary consumers. In all, the sum total of the energy remains constant.

b. Using the second law: Flow of energy cross different levels in a food chain .Transfer of energy between trophic levels is not 100% completely efficient successive levels have less energy and support few organisms, Primarily producers or green plants have the highest energy. When herbivores or primary consumers feed on primary producers, the energy level is reduced. When carnivores or secondary consumers feed on herbivores, the energy level is further reduced. Part of the energy is lost as heat at each trophic level.Not all parts of the preceeding organisms are eaten by the predator or organism at the next trophic level. Energy is lost in the process of respiration or feeding or movement or metabolic activities.

Pyramid Of Energy

(a) Using The first law: It is discovered that energy is transferred from one trophic level to another. The energy of the producers at the base of the pyramid is higher and it is transformed gradually from one stage of the trophic level to another. Even though energy is transformed into various other forms in the successive trophic levels, the sum total of the energy is cinstant.

(b) Using the second law: As the energy is transformed from one trophic level to another. Part of it is converted into heat which is lost, causing a progressive drop in energy in successive trophic levels.

Energy Flow

(a) Using the first law: As the producers convert the solar energy to useful energy, this energy is progressively transformed from one trophic level to the next and so on. This means , energy flows from producers to primary consumers , then to secondary consumers and finally to tertiary consumers in a food chan. So the flow of energy in a food chain is in one direction only.

(b) Using the second law: The transfer of energy between trophic levels is not 100% or it is not completely transferred.Successive levels have less of useful energy and support fewer organisms or individuals lives. Primary producers to plants have the highest amount of energy. When herbivores feed on plants , the energy level is reduced. When carnivores consume the herbivores , the energy level is further reduced.

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Take a quick test for this lesson

1. How does energy living organisms carried out energy transmission?
2. Explain the flow of energy
3.How does energy lost in the ecosystem?
4. State the first and second laws of thermodynamics (b) How can the laws be applied to (i) food chain (ii) pyramid of energy (iii) energy flow
5.(a) What is meant by the term food chain?
(b) Draw a food chain involving four trophic levels which can be found in a terrestial habitat. (c) Explain (i) the flow of energy through the food chain drawn in 3(b) above (ii) how energy in the chain is lost to the environment.

6(a) State the second law of thermodynamics (b) Use the second law of energy across different tropic levels in a food chain

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